look for something like this in details.php
PHP Code:
<td style="padding-left: 5px;" class="subheader" valign="top" width="147">'.$lang->details['download'].'</td>
and add this under it
PHP Code:
<td style="padding-left: 5px;" valign="top" width="430"><a href="'.ts_seo($id,$torrent['name'],'d').'" alt="'.$lang->details['dltorrent'].'" title="'.$lang->details['dltorrent'].'"><img src=download.png></a></td>
Then create or find a icon and name it download.png and put it in your www root